In reproductive tissues such as the breast and the uterus, cell proliferation and differentiation is strongly regulated by complex interactions between estrogen receptor alpha (ERalpha) and growth factor receptors. In the present study, we investigated the potential occurrence of such cross-talk in the murine, gonadotropic alphaT3-1 cell line, which expresses ERalpha and the IGF-I receptor (IGF-IR). Under estrogen-free conditions, basal cell proliferation and ER-mediated gene transcription was strongly inhibited by the selective estrogen receptor modulator (SERM) 4-hydroxy-tamoxifen (4-OH-Tam) and by the pure anti-estrogen ICI 182,780 (ICI). These effects can be reversed by either 17-beta-estradiol (E(2)) or insulin-like growth factor I (IGF-I), both exerting modest mitogenic effects in the alphaT3-1 cell line. Furthermore, IGF-I enhanced both basal and E(2)-induced ER-driven gene transcription. This may be explained, at least in part, by enhanced phosphorylation of ERalpha at serine 118, a prerequisite for the transactivation capacity of the receptor. Finally, the IGF-I-induced response on cell growth and ER-mediated transactivation can be inhibited with either ICI or 4-OH-Tam. In conclusion, our data indicate IGF-IR and ER interactions in the alphaT3-1 cell line, an in vitro model for the pituitary gonadotrophs, hereby suggesting a role of IGF-I in the regulation of gonadotropin synthesis and secretion.